Because of their very large bandwidth capacity, optical signals are being increasingly utilized for the transmission of data. Further, the bandwidth capacity of a given fiber optic cable can be further increased by transporting a multiplicity of independent signals within a single fiber on separate channels at slightly different wavelengths, a technique known as wavelength division multiplexing (WDM). Thus, for example, a nominally 1550 nm fiber optics signal might comprise four, eight, 64, 80 or more channels, each separated by for example approximately 0.8 nm (corresponding to 100 GHz) or approximately 1.6 nm (corresponding to 200 GHz). Multiwavelength operation facilitates an increasingly important advantage of optical transport and switching which is that several non-interacting signals may pass through the switch simultaneously, which signals convey entirely incompatible data rates, encodings and protocols in parallel without compromising one another. However, for such signals to be useful, it must be possible to wavelength selectively switch the optical signals coming in on an optical fiber, bus (or other optical conduit) to a fiber/conduit leading to a desired drop/destination, to wavelength selectively add signals from a drop to the bus or to wavelength selectively transfer signals between fibers or other optical conduits. The first two functions are sometimes referred to as switchable add/drop filtering (SADF) and the last function is sometimes called wavelength selective cross-connect (WSXC). In other applications, switching the entire fiber signal, inclusive of all wavelength channels, is required (such switching sometimes being denoted as “space switching”). In complex fiber optic structures such as those used in the telecommunications industry and for sensor and computer data networks, light signals must be efficiently routed or switched from an array of N incoming optical fibers, which fibers may be single mode or multimode, to an array of M outgoing optical fibers. Such a space switch will sometime be referred to hereinafter as an NXM switch or cross-connect.
While a number of techniques have been proposed over the years for performing NXM switching optically, none of these techniques have proved to meet all requirements simultaneously. This is partly due to the varied architectures which are required for such switches. For example, an N fiber in, N Fiber out (NXN) switch that maps each incoming fiber optical signal to one and only one fiber output is termed an NXN cross-connect. It is nonblocking if any connection is possible, without regard to earlier established connections. For some applications, reconfigurably nonblocking switches are sufficient. In other applications, switches that multicast or broadcast, sending one incoming signal to more than one output, or that perform other variant functions, are required. The data capacity demands on fiber optic networks are also becoming more complex, imposing a requirement that switching technologies be scalable so as to be extendable in a straight forward manner from small switches (for example 2×2 or 4×4 to larger switches such as 64×64, 1024×1024, and beyond). It is also desirable that such switches be integrable such that individual miniaturized switching elements can be combined with many others on a single chip or substrate to provide a larger NXN or NXM cross-connect structure. However, designing such structures, particularly for larger switches, is very complex even for single channel operation, and the complexity increases dramatically for multichannel WDM operation (i.e., wavelength selective switching with an N×M×m switch, where m is the number of WDM channels).
Another requirement for optical switches of the type described above in particular, and for optical components and structures in general, is that they efficiently interface with optical fibers, the use of which to transport high bandwidth signals over long distances is increasingly prevalent, in a manner so as to minimize coupling losses. Other key performance parameters include minimizing insertion loss, crosstalk and polarization sensitivity, insuring good optical isolation in all switch states, good spectral bandwidth, and good dynamic range for on/off contrast ratio. Low operating power, high switching speed, low power consumption, stability, long service life/temperature insensitivity and high reliability are also important. However, for many network reconfiguration and protection switching functions, switching speeds in the range of 1 microsecond to 1 millisecond are adequate and sufficient.
Further, in the present state of the art, neither space switching, nor wavelength selective switching techniques, are entirely satisfactory. One reason for this is that the various network control and reconfiguration functions required have generally been met by different and incompatible technologies. Optical network systems would be considerably advanced, in efficiency, manufacturability and cost, if several disparate network control functions could be implemented on the basis of a single underlying technology.
All-optical switching is increasingly regarded as essential for future networks. Because satisfactory products for performing such optical switching have not existed, it has therefor been necessary to convert optical signals to be switched into electrical signals for switching and to then reconvert the signals to optical signals for outputting. This technique can be expensive, time consuming, impose bandwidth limitations on the system and introduce several sources of potential error. It can also limit the flexibility of the system and is generally not an efficient way to operate.
In addition to the switching applications discussed above, there are numerous applications where a need exists to be able to change the direction in which an optical signal is passing through a waveguide, dynamically filter an optical signal, particularly a multiwavelength or multichannel signal, so as to selectively add, drop, pass or block various of the individual wavelengths or channels (or the entire signal), to selectively attenuate an optical signal, including one or more signals of a multiwavelength or multichannel line, to selectively crossconnect optical paths including multichannel or multiwavelength optical paths to facilitate the transfer of one or more channels therebetween, and/or to selectively couple the multiwavelengths or multichannels along optical paths out of the plane of the waveguide. It should be possible to perform all of these functions utilizing optical components and/or structures which are relatively easy and inexpensive to fabricate. In particular, it would be desirable if fabrication techniques could be provided which would permit complex optical networks to be fabricated utilizing a parallel, simultaneous, one-shot fabrication techniques that incorporates a multiplicity of functionalities on a single chip for the implementation of space switching, wavelength selective switching, switchable add-drop filtering, wavelength selective cross-connect switching, together with such additional functions as programmable attenuation, all on a single chip and using a single material technology rather than requiring each component to be separately fabricated.